This invention relates to a navigation system to be mounted on a vehicle.
Japanese Patent Publication No. 61275/87 discloses a navigation system comprising a microcomputer, an angular velocity sensor, and a travel sensor. The angular velocity sensor outputs a voltage that corresponds to an angular velocity of a vehicle generated when the vehicle changes its direction. The output voltage from the angular velocity sensor is converted to a digital signal by an analog-to-digital converter. The travel sensor sends a pulse every time the vehicle travels a predetermined distance. The microcomputer receives the digital angular velocity; calculates, from the pulses coming from the travel sensor, the distance the vehicle has travelled; and then computes a current position of the vehicle from the angular velocity and distance travelled.
Even if the angular velocity of the vehicle is constant, the output of the angular velocity sensor varies slightly with temperature and other conditions. Thus the navigation system disclosed in the above publication requires a correction circuit. This correction circuit takes the output voltage of the angular velocity sensor when the vehicle is stopped as a reference voltage for zero angular velocity. And the correction circuit uses the reference voltage to correct the output of the analog-to-digital converter to a presumptively accurate angular velocity. The correction circuit works by monitoring the pulse from the travel sensor and resetting the reference voltage after a predetermined interval of no pulse. This procedure of resetting the reference voltage continues until immediately before the pulse from the travel sensor resumes.
However, the navigation system disclosed above has disadvantages. When the vehicle is moving slowly, the travel sensor may send a pulse later than when the vehicle actually starts to move. If the vehicle changes direction as it starts to move, before the pulse is sent, the correction circuit takes the output voltage of the angular sensor as the no-motion voltage and resets the reference voltage, even though the angular velocity of the vehicle is not zero. Because the correction circuit has reset the reference voltage to the output voltage when the angular velocity is not zero, it becomes practically impossible to determine the angular velocity accurately thereafter. Moreover, the reference voltage, reset to the output voltage from the angular velocity sensor immediately before the pulse is sent, may not be reliable, because a number of factors can cause the output voltage from the angular velocity sensor to vary.